I Field of the Invention
This invention relates generally to rotary cone rock bits having hard metal cutter inserts mounted on the rotary cones, and more specifically to chisel type inserts used particularly on the gage row of each cone.
II Description of the Prior Art
There are a number of prior art patents that disclose chisel inserts used on the gage row of rock bit cones. For example, U.S. Pat. No. 3,442,342 discloses a gage insert having flats ground thereon to the precise gage diameter of the borehole. FIGS. 15 and 16 of the patent show such an insert that is illustrated in FIGS. 1-4 of the present specification. As described, these flats provide a relatively large contact area against the borehole sides. However, the problem with such inserts is that the large contact areas are susceptible to heat checking, resulting in premature wear and breakage. Insert heat checking can be defined as high cycle thermal fatigue due to intermittent frictional heat generated by borehole wall to gage insert contact and subsequent cooling by drilling fluid per each revolution.
U.S. Pat. No. 4,832,139 shows an inclined chisel insert having different cone angles on opposite sides of the crest. The advantage of such an insert is that it provides a relatively small area of contact with the borehole wall thereby being less prone to frictional heating.
Another type of insert is shown in U.S. Pat. No. 4,086,973. Although this insert is not a gage insert, it does show an inclined crest positioned to contact the formations with substantially its entire length.
All of the above cited inserts are designed with a constant radius joining the crest and the flanks. This leaves a flat on top of the insert to do some cutting when the flanks of an insert have no common plane perpendicular to both. This flat is not optimal as the constant radius that joins the flat to the flanks is an area of high stress concentration.
However, when the flanks of the insert have a common plane perpendicular to them both, the end of the crest that cuts the hole wall generally tends to wear quickly due to the reduced volume of insert material in that region.
These prior art designs also had equal outer and inner corner radii extending beyond the ends of the crests. This type of structure causes the outer ends of the inserts to wear faster than otherwise desirable, therefore leading to premature undergage conditions.